Multi-layer structure with variable bandpass for monochromatization and spectroscopy

ABSTRACT

A grating that includes a multilayer structure that has alternating layers of materials, a plurality of grooves formed between a plurality of lands, wherein at least one structural parameter of the plurality of grooves and plurality of lands is formed randomly in the multilayer structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to multi-layer gratings/mirrors and theirapplication in analyzing systems.

2. Discussion of Related Art

It is well known in the art to use large d-spacing artificially growncrystals, e.g. PET crystals, for x-ray fluorescence spectroscopy. Suchartificial crystals are very sensitive to the environment and degradequickly under the influence of radiation, heat, moisture, etc.Accordingly, the artificial crystals do not last long and are difficultto use. In hard x-rays region (wavelength less than 0.2 nm) ordinarycrystals, e.g. Si or Ge crystals, when used in monochromators andspectrometers in some cases have resolving powers that are too high(˜104) and cut out too much useful flux. Accordingly, the measurementtimes become longer.

While reflective gratings can be used in place of the artificialcrystals, such gratings suffer from low diffraction efficiency in thesofter x-ray region (wavelengths ranging from about 0.2 nm to about 1.2nm). Multilayer mirrors, while improving reflection efficiency relativeto the gratings in a wide wavelengths range (from 1 nm to 20 nm), have aresolving power that is too low ˜λ/Δλ˜10-100.

Multilayer gratings/mirrors are well known in the art and are verystable, durable and are easy in use. Examples of such multilayergratings/mirrors are described in U.S. Pat. Nos. 4,727,000; 5,646,976and 5,757,882, the entire contents of each are hereby incorporatedherein by reference. It is well known that the bandpass of suchmultilayer gratings/mirrors is defined by the number of bilayers inwhich the incident wave penetrates. This number of bilayers in themultilayer grating/mirror is limited by the factor that due tointerference in, the periodical structure of the multiple layers stackedupon one another, the radiation wave incident on the multilayergrating/mirror is reflected back and does not penetrate any deeper thana so-called extinction depth. The extinction depth is determined by thewavelength of the incident radiation and the materials of the multilayergrating/mirror.

The bandpass and correspondingly the resolution of a spectrometer or amonochromator that uses a multilayer grating/mirror is proportional to:

Δλ/λ˜1/N,  (1)

wherein N is the number of multilayer periods present within theextinction depth. In many instances, the resolution of a spectrometer ora monochromator is required to be better than that as determined by theextinction depth in the manner described above.

Based on the above relationship, one way to increase the resolution of amultilayer grating/mirror is to increase the extinction depth and thusthe number N of multilayer periods within the extinction depth. Oneknown way to increase the extinction depth is to etch grooves in themultilayer grating/mirror and remove part of the reflection planes so asto allow the incidence radiation wave to penetrate deeper into themultilayer grating. As a result, the number of layer N in the extinctiondepth increases and the bandpass, or the resolution, increases inaccordance with equation (1) above. Such a multilayer grating/mirror isdiscussed in the paper entitled “Lamellar Multilayer Gratings with VeryHigh Diffraction Efficiency,” V.V. Martynov et al., SPIE Vol. 31500277-786X/97, pp. 2-8.

By changing the groove/period ratio of the multilayer grating/mirror,the amount of removed material can be continuously varied and, thus, theextinction depth and resolution can be continuously varied. The maximumpractical factor in the bandpass variation is defined by technologicallimits and, for different wavelength, can be as from 1 to 100. Whilesuch a multilayer grating/mirror provides increased resolution, themultilayer grating/mirror with periodically spaced lands also generatesmany diffraction orders, which contribute in making the detector signalto have a small signal to noise ratio. The generation of multiplediffraction orders is shown by analogy to the single layer periodictransmission grating diffraction intensity distribution shown in FIG. 3.Obviously, if a single layer transmission grating with a periodicstructure generates multiple diffraction orders, then a multi-layertransmission grating with a periodic structure will also generatemultiple diffraction orders.

Accordingly, it is an objective of the present invention to provide amultilayer grating/mirror that for a wide range of wavelengths hasincreased resolution and diffraction efficiency while at the same timecontributing in making the detector signal having a large signal tonoise ratio.

Another object of the present invention is to provide crystals that arenot sensitive to the environment and do not degrade quickly when used inan analyzing system.

SUMMARY OF THE INVENTION

One aspect of the present invention regards an analyzing system thatincludes a radiation generator that generates a beam of radiation alonga first direction and a grating that receives the beam of radiation andgenerates a second beam of radiation that possesses only a zeroth orderof diffraction. An object receives the second beam of radiation andgenerates a third beam of radiation and a detector system that receivesthe third beam of radiation.

A second aspect of the present invention regards a grating that includesa multilayer structure that has alternating layers of materials, aplurality of grooves formed between a plurality of lands, wherein atleast one structural parameter of the plurality of grooves and pluralityof lands is formed randomly in the multilayer structure.

An advantage of each aspect of the present invention is to providecrystals that are not sensitive to the environment and do not degradequickly when used in an analyzing system.

A second advantage of each aspect of the present invention to provide amultilayer grating/mirror that has increased resolution and diffractionefficiency while at the same time contributing in making the detectorsignal having a large signal to noise ratio.

Additional objects and advantages of the invention will become apparentfrom the following description and the appended claims when consideredin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an embodiment of an x-ray fluoroscopy systemaccording to the present invention;

FIG. 2 schematically shows a side cross-sectional view of an embodimentof a multilayer grating/mirror according to the present invention to beused with the x-ray fluoroscopy system of FIG. 1;

FIG. 3 shows a diffraction intensity distribution from a periodicalgrating, giving many diffraction orders; and

FIG. 4 shows a diffraction intensity distribution from a single layergrating with random line spacing where all diffraction orders aresuppressed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, an x-ray fluoroscopy analyzing system 10 includes aradiation generator or source, such as x-ray tube 12, that generates abeam of radiation along a first direction, such as a beam of x-rays 14.The x-rays 14 have a wavelength that ranges from 0.3-1.0 nm. The x-rays14 generated from x-ray tube 12 are received by and interact with theobject or sample 16 so that x-ray fluorescence radiation 18 is generatedfrom the object 16. The x-rays 18 are directed through a slit 19 andreceived by a multilayer grating/mirror 20, which reflects only a zerothorder of diffraction of x-rays 22 of a particular wavelength, such as0.71 nm. The x-rays 22 are then received by a detector system 24, suchas a proportional counter detector. The detected radiation is thenanalyzed in a well known manner.

As shown in FIG. 2, the multilayer grating/mirror 20 includes amultilayer structure 26 deposited on a substrate 28. The multilayerstructure 26 is made out of alternating layers of materials with largeand small atomic numbers. The material with large atomic number can beselected from the materials W, Ni, Fe, Mo, V, Cr and the material withsmall atomic numbers can be selected from the materials C, Si, B4C. Forexample, the multilayer structure 26 can be made out of alternatinglayers of W (10 Å) and C (10 Å) layers. Thus, the period, d, of thealternating W and C layers is 20 Å. In this embodiment, the number ofperiods, d, of alternating W/C bi-layers in the multilayer structure 26is 500. Note that the number of bi-layer depends on a spectralresolution/bandpass requirements. For 500 bi-layers, the bandpassλ/Δλ˜N˜500. The period of the multilayer depends on a required Bragg angle andtypically ranges from 15 A to 100 A for different wavelengths. Inaddition, other materials and thicknesses for the layers of materialswith large and small atomic numbers are possible depending on thespecific needs for wavelength and Bragg angle.

As shown in FIG. 2, a plurality of grooves 30 are formed randomly on themultilayer grating/mirror 20. The grooves 30 are positioned betweenlands 32 of the multilayer structure 26, wherein each land 32 has awidth of approximately 1 micron and contains 500 periods of alternatingW/Si bilayers. The starting points or positions xi of the lands 30 canbe determined by a formula given below:

xi=(d*i)+[ki*(d−Wland)],   (2)

where d=effective period of the grating/mirror 20, i=1, 2, 3, . . . ,;Wland=width of land and ki=a random number from 0 to 1. Note that thewidths of the lands and depths of the grooves are constant for theentire area of the grating. Furthermore, the lands are placed randomlyinside each period according to the formula (2) above.

One of the benefits of using a multilayer grating/mirror 20 with arandom pattern of grooves is that all diffraction orders, except thezeroth order, are suppressed. In other words, only the direct beam isreflected by the grating/mirror 20. The suppression of diffractionorders is shown by analogy to the single layer random structuretransmission grating diffraction intensity distribution shown in FIG. 4.Obviously, if a single layer transmission grating with a randomstructure suppresses multiple diffraction orders, then a multi-layertransmission grating with a random structure will also suppress multiplediffraction orders.

In the above-described mode of randomizing the grating/mirror 20, theland widths and the grooves depths are selected so that a desired widthof the peak of the rocking curve of the grating/mirror 20, which is thesame as an energy bandpass or spectral resolution of the grating/mirror20, is achieved. Thus, the ability to change the bandpass allows thespectral resolution to be adjusted to specific requirements and so as tooptimize flux and resolution.

While the above description constitutes the preferred embodiments of thepresent invention, it will be appreciated that the invention issusceptible of modification, variation and change without departing fromthe proper scope and fair meaning of the accompanying claims. Forexample, the grating 20 can also be used as a monochromator.

We claim:
 1. An analyzing system comprising: a radiation generator thatgenerates a beam of radiation along a first direction; an object thatreceives said beam of radiation and generates a second beam ofradiation; a grating that receives said second beam of radiation andgenerates a third beam of radiation that possesses only a zeroth orderof diffraction; and a detector system that receives said third beam ofradiation.
 2. The analyzing system of claim 1, wherein said radiationgenerator comprises an x-ray source that generates a beam of x-rays. 3.The analyzing system of claim 1, wherein said grating comprises amultilayer structure.
 4. The analyzing system of claim 3, wherein saidmultilayer structure comprises alternating layers of materials withlarge and small atomic numbers.
 5. The analyzing system of claim 4,wherein said material with a large atomic number is tungsten and saidmaterial with a small atomic number is silicon.
 6. The analyzing systemof claim 3, wherein said multilayer structure comprises a plurality ofgrooves formed between a plurality of lands.
 7. The analyzing system ofclaim 6, wherein each of said lands has an equal number of layers ofsaid multilayer structure.
 8. The analyzing system of claim 6, whereinsaid plurality of lands are formed randomly in said multilayerstructure.
 9. The analyzing system of claim 8, wherein each of saidplurality of lands has a constant width.
 10. The analyzing system ofclaim 8, wherein the depth of each of said plurality of grooves isconstant.
 11. The analyzing system of claim 9, wherein the depth of eachof said plurality of grooves is constant.
 12. The analyzing system ofclaim 8, where the starting positions xi of said plurality of lands aredetermined by the formula given below: xi=(d*i)+[ki*(d−Wland)], whered=period of said grating, i=1, 2, 3, . . . ; Wland=width of land; andki=a random number from 0 to
 1. 13. The analyzing system of claim 12,wherein the widths of each of said lands is constant.
 14. The analyzingsystem of claim 12, wherein the widths of each of said lands isconstant.
 15. The analyzing system of claim 13, wherein the widths ofeach of said lands is constant.
 16. A grating comprising: a multilayerstructure comprising: alternating layers of materials; a plurality ofgrooves formed between a plurality of lands, wherein at least onestructural parameter of said plurality of grooves and plurality of landsis formed randomly in said multilayer structure.
 17. The grating ofclaim 16, wherein said alternating layers of materials comprisealternating layers of materials with large and small atomic numbers. 18.The grating of claim 17, wherein said material with a large atomicnumber is tungsten and said material with a small atomic number issilicon.
 19. The grating of claim 16, wherein each of said lands has anequal number of layers of said multilayer structure.
 20. The grating ofclaim 16, wherein said at least one parameter is the location of saidplurality of grooves.
 21. The grating of claim 20, wherein each of saidplurality of lands has a constant width.
 22. The grating of claim 20,wherein the depth of each of said plurality of grooves is constant. 23.The grating of claim 21, wherein the depth of each of said plurality ofgrooves is constant.
 24. The grating of claim 20, where the startingpositions xi of said plurality of lands are determined by the formulagiven below: xi=(d*i)+[ki*(d−Wland)], where d=period of said grating,i=1, 2, 3, . . . ; Wland=width of land; and ki=a random number from 0to
 1. 25. The grating of claim 24, wherein the widths of each of saidgrooves is constant.
 26. The grating of claim 24, wherein the widths ofeach of said lands is constant.
 27. The grating of claim 25, wherein thewidths of each of said lands is constant.